The invention relates to a simple projection imaging technique and apparatus, suitable for use with all forms of radiation such as visible, deep ultraviolet (UV) and x-rays, and particularly suitable for the replication (i.e., "lithography") of high density integrated circuit Patterns. The invention particularly relates to reduction or elimination of distortion problems in large field, high resolution lithography.
A number of different lithographic techniques are presently used for semiconductor microcircuit manufacturing. For example, visible and UV radiation are used in steppers to transfer a pattern from a master reticle to a wafer. This process uses projection optics and often demagnifies the image of the master pattern onto the wafer. To obtain a large, nearly distortion-free image field (the maximum allowable distortion in the image is typically required to be less than one tenth of the minimum linewidth in the entire field) with high spatial resolution, complex imaging systems are needed. These systems often use more than two optical elements and more than one aspheric optical element. Current commercial systems have a spatial resolution (i.e., can image a minimum linewidth) of approximately 0.5 microns and expose fields of approximately 25 mm in diameter. Due to fundamental physical constraints, this process operating at wavelengths greater than 190 nm will be unable to produce circuits with minimum feature sizes much smaller than approximately 0.2 microns.
In proximity print x-ray lithography (PPXRL), e.g., as described by A. Heuberger, J. Vac. Sci. Tech., B6, 107 (1988), a mask and wafer are placed in "near contact" to one another and the pattern on the mask is replicated onto the wafer by "shadow-casting". The mask is fabricated on a thin (less than 5 micron thick) membrane, is difficult to manufacture and is subject to distortion. Because of diffraction, PPXRL may not be able to achieve very high resolution (i.e., PPXRL may be limited to linewidths greater than 0.2 micron).
Soft x-ray projection lithography (SXPL), e.g., as described by A. M. Hawryluk and L. G. Seppala, J. Vac. Sci Tech., B6, 2162 (1988) and in U.S. patent application Ser. No. 308,332 filed Feb. 9, 1989 by A. M. Hawryluk and L. G. Seppala, has been shown to be capable of very high resolution (e.g., minimum feature size down to 0.05 microns or less), but requires a totally reflective imaging system. A reflective imaging system requires a multi-element, aspheric optical imaging system to achieve large flat fields (greater than 25 mm in diameter) with low distortion. This optical system requires very precise components which have yet to be developed. An alternative to this complex reflective imaging system utilizes an imaging system with all spherical reflective components and a curved reflecting mask. The advantage of this system is the ease of fabrication of spherical optics (relative to aspherical optics), but the disadvantage is the introduction of field curvature and distortion to the imaging system. Field curvature in the spherical imaging system can be eliminated with the appropriate mask curvature, and field distortion can be eliminated by patterning the curved mask in a manner that exactly compensates the distortion in the imaging system. For this approach to work, precise spherical optical imaging elements and apriori knowledge of the field distortion are required. In addition, mask patterning, repair and inspection techniques on curved substrates have not yet been demonstrated.
Conventional microcircuit mass production techniques are used to image or shadow-cast a master pattern (the reticle) onto a wafer. The process takes a precise, flat reticle and transports the image through a precise optical system (i.e., the optical transport system is nearly distortion free) to produce the desired pattern on the flat wafer. The requirement that the optical transport system be nearly distortion free and have a flat field (at both the reticle and the wafer) greatly increases the complexity of conventional lithographic tools.